1. Field of the Invention
This invention relates to an objective lens driving apparatus provided on an optical disc apparatus, and more particularly to an objective lens driving apparatus that not only switches objective lenses with different numerical apertures according to the type of a recording medium but also identifies the type of the selected objective lens.
2. Description of the Related Art
With the recent development of optical information recording mediums, including optical discs and magneto-optical discs, objective lens driving apparatuses for use with the reproduction systems of those optical information recording mediums are actively being developed. Objective lens driving apparatuses have been widely used as compact disc (CDs) or CD-ROM driving apparatuses.
Recently, objective lens driving apparatuses have been developed for not only reproducing use but also recording use. Well-known recording methods used for those apparatuses are the magneto-optical recording method and the phase-modulation recording method. Many of such recording methods have been prescribed in detail by standards. In recent years, however, a high-density recording optical disc aimed at improvements in the recording density has appeared and the research and development of optical discs of the high-density recording type have been done at a rapid rate. With such optical discs, for high-density recording, pits serving as data recording units are required to be made smaller than those on conventional CDs and the pits are needed to be searched for with high accuracy. An optical disc of the high-density recording type differs from a conventional CD in the thickness of the substrate. For an apparatus for reproducing the optical disc, the wavelength of a laser beam searching for pits is shorter and the numerical aperture of the objective lens is set larger so that the diameter of the beam spot formed on the optical disc may be smaller.
When various modification are made on new discs appearing one after another as described above, it is difficult for apparatuses of the above-described type to record and reproduce the data onto and from optical discs complying with the conventional standards. It is an inconvenience to the users to prepare a disc apparatus according to the recording medium used.
To solve such a problem, a method of providing a plurality of optical heads with different focal lengths on a single optical disc apparatus has been proposed, as disclosed in U.S. Pat. No. 5,235,581. With the disc apparatus, two optical heads are provided so as to enable tracking independently, thereby making it possible to record and reproduce the data onto and from a conventional compact disc, such as a compact disc.
In such a method, two optical heads are positioned so that they may face each other symmetrically with the center of the optical disc and the two optical heads cannot be placed side by side. Therefore, with an optical disc apparatus employing such a method, in the case of an optical disc (e.g., a CD ROM or MO) housed in a cartridge (caddie) with a window section, none of the two objective lenses can be positioned under the opening of the window section with a limited area. As optical disc apparatuses have been popularized, there have been great demands toward lower-cost apparatuses. The need for two optical heads, however, is an obstacle to such demands.
From this viewpoint, it is hoped that an optical head in which two or more objective lenses of different types are provided and which switches the objective lens therein will appear. With this configuration, the development of an optical head with a structure that can identify the type of the selected objective lens is also wanted.